1. Technical Field of the Invention
The present invention relates generally to a spark plug in which sparks are produced between a second ground electrode and a center electrode when a porcelain insulator is stained with carbon, and more particularly to an improved structure of such a type of spark plug designed to provide higher wear resistance to a center electrode for increasing the useful life of the spark plug and a production method thereof.
2. Background Art
European Patent Application EP 1 006 631 A2 discloses a conventional spark plug of a type in which production of electric sparks is initiated between a second ground electrode and a center electrode when a porcelain insulator is stained with carbon. FIG. 10 shows a typical structure of such a type of spark plug.
The spark plug includes a porcelain insulator 2 installed within a metal shell (not shown). Within the porcelain insulator 2, a center electrode 3 is disposed which has a tip 3a projecting from a tip 2a of the porcelain insulator 2. A noble metal chip 3d is welded to the tip 3a of the center electrode 3.
The spark plug also includes a first ground electrode 4 and a pair of second ground electrodes 5 (only one is shown for the brevity of illustration) which are installed on an end of the metal shell. The first ground electrode 4 is opposed at an end thereof to the noble metal chip 3d of the center electrode 3 to define a spark gap. The second ground electrode 5 has an end facing an end portion of a side surface of the center electrode 3 exposed outside the tip 2a of the porcelain insulator 2.
In operation, electric sparks are generated sequentially between the first ground electrode 4 and the center electrode 3 to ignite a gaseous fuel such as an air-fuel mixture injected into an internal combustion engine. When the fuel is burned, it will cause carbon to stick to the surface of the tip 2a of the porcelain insulator 2, thereby resulting in a decreased degree of electric insulation of the porcelain insulator 2. This causes sparks to be initiated between the second ground electrode 5 and the center electrode 3, thereby burning away the carbon adhered to the porcelain insulator 2. When the surface of the porcelain insulator 2 is cleaned of the carbon, it will cause sparks to be generated again between the first ground electrode 4 and the center electrode 3.
In order to improve the effects of burning the carbon away from the porcelain insulator 2, a shoulder 3b is formed on the center electrode 3 which tapers off to the tip 3a. A boundary 3c between a major portion of the center electrode 3 and the shoulder 3b is located inside the porcelain insulator 2.
The boundary 3c forms a corner on which an electric field is concentrated. Sparks, thus, fly, as indicated by an arrow in the drawing, over the tip 2a of the porcelain insulator 2 between the boundary 3c and the end of the second ground electrode 5, which serves to burn the carbon away from the porcelain insulator 2 effectively.
Researches carried out by the inventors of this application, however, showed that even though the carbon does not stick to the surface of the porcelain insulator 2, sparks may be produced between the second ground electrode 5 and the center electrode 3 depending upon specifications and/or operating conditions of the engine.
The production of sparks between the second ground electrode 5 and the center electrode 3 when there is no carbon sticking to the porcelain insulator 2 will cause a portion, as indicated by S in the drawing, of the side wall of the center electrode 3 to be worn or scooped away, thus resulting in scattering of metallic components of the center electrode 3 onto the surface of the porcelain insulator 2. When the metallic components are deposited on the porcelain insulator 2, it facilitates ease of production of sparks between the second ground electrode 5 and the center electrode 3, thus increasing wear of the side wall of the center electrode 3 undesirably.